Gain controller using switched capacitors

ABSTRACT

A gain controller using switched capacitors is provided. The gain controller is operable in a sampling mode or an amplifying mode and controls the gain of an analog input signal. The gain controller includes an operational amplifier, input capacitors, a feedback capacitor, and switches. The operational amplifier controls the gain of the analog input signal and generates an output signal having the controlled gain. The input capacitors are connected to the input side of the operational amplifier in parallel. The feedback capacitor is connected between the input side and the output side of the operational amplifier. The switches connect at least one of the input capacitors to the input signal or a reference voltage, in response to the kinds of operation modes and a predetermined externally applied digital gain control signal. According to the gain controller using switched capacitors, it is possible to automatically control the gain of an input signal at high speed and to reduce power consumption since the time required for settling the gain of the input signal to a desired value is short.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gain controlling circuitry, and moreparticularly, to a gain controller for digitally controlling the gain ofan input signal using switched capacitors.

2. Description of the Related Art

FIG. 1 is a block diagram of a conventional gain controller forcontrolling the gain of an input signal using switched capacitors. Thegain controller of FIG. 1 includes feedback capacitors C_(F1) andC_(F2), an operational amplifier 10, input capacitors C_(I1) and C_(I2),and MOS transistors MN1 through MN7 which operate as switches.

FIG. 2 shows waveforms of first and second control clock signals Q1 andQ2 for controlling the opening and closing of the MOS transistors MN1through MN7 shown in FIG. 1, which operate as the switches. The clocksignals Q1 and Q2 of FIG. 2 are applied to MOS transistors MN1 throughMN7 as shown in FIG. 1. Specifically, Q1 is used to control MOS atransistors MN3 through MN7, and Q2 is used to control MN1 and MN2. Itis noted that the falling edge of the first control clock signal Q1 doesnot overlap the rising edge of the second control clock signal Q2. Thisis to prevent the MOS transistors MN1 and MN2 and the MOS transistorsMN3 through MN7 from being simultaneously turned on.

The gain controller shown in FIG. 1 operates in two modes. Namely, thegain controller operates in a sampling mode when the first control clocksignal Q1 is at a logic “high” level and operates in an amplifying modewhen the second control clock signal Q2 is at the logic “high” level.

Charge amounts Q_(I1) and Q_(I2) charged to the input capacitors C_(I1)and C_(I2), respectively, in the sampling mode are equal to the sumsQ_(f1) and Q_(f2) of charge amounts charged to the input capacitorsC_(I1) and C_(I2) and the feedback capacitors C_(F1) and C_(F2),respectively, in the amplifying mode. When the capacitance of the inputcapacitors C_(I1) and C_(I2) is C_(I) and the capacitance of thefeedback capacitors C_(F1) and C_(F2) is C_(F), the followingrelationship is established between the difference V_(INT)−V_(INC)between input voltages V_(INT) and V_(INC) and the differenceV_(OUTT)−V_(OUTC) between output voltages V_(OUTT) and V_(OUTC).$\begin{matrix}{{V_{OUTT} - V_{OUTC}} = {\frac{C_{I}}{C_{F}}\left( {V_{INT} - V_{INC}} \right)}} & (1)\end{matrix}$

A feedback factor is 1/β in the amplifying mode. Here, β is a feedbackgain value, which is represented as follows. $\begin{matrix}{\beta = \frac{C_{F}}{C_{I} + C_{F}}} & (2)\end{matrix}$

As the above-mentioned feedback factor 1/β increases, the operatingspeed of the gain controller shown in FIG. 1 is reduced. Therefore, inorder to increase the operating speed of the gain controller, the valueof β defined by Equation 2 must be increased.

It is assumed that the MOS transistors MN1 and MN3 and the inputcapacitor C_(I1) shown in FIG. 1 constitute a cell. A plurality of cellsare connected to the negative input terminal of the operationalamplifier 10 in parallel. Also, it is assumed that the MOS transistorsMN2 and MN4 and the input capacitor C_(I2) constitute another cell. Aplurality of cells are connected to the positive input terminal of theoperational amplifier 10 in parallel. In order to select desired cellsamong the cells connected An parallel, an externally generated digitalgain control signal is used. Accordingly, the value of the gainC_(I)/C_(F) shown in Equation 1 can be changed.

In the conventional gain controller, the digital gain control signal isused in order to change the value of C_(I)/C_(F). Only one cell isselected. The input capacitance of the capacitor included in theselected cell is previously set so as to obtain a desired value ofC_(I)/C_(F). Namely, when the gain C_(I)/C_(F) of the differenceV_(INT)−V_(INC) is to be controlled as a C_(I)/C_(F) in the conventionalgain controller, corresponding two cells, which include the capacitorhaving the capacitance a*C_(F)′ (C_(F)′ is the capacitance of C_(F)),are to be selected in response to the digital gain control signal amongthe plurality of cells connected to the positive and negative inputterminals of the operational amplifier 10 in parallel.

For example, it is assumed that the capacitance of the feedbackcapacitor C_(F) is set to be C_(O) and that first through fourth cellsare connected to the negative input terminal of the operationalamplifier 10 in parallel. Also, it is assumed that fifth through eighthcells are connected to the positive input terminal of the operationalamplifier 10 in parallel and that the capacitor included in the firstcell and the capacitor included in the fifth cell have the capacitanceC_(O). Also, it is assumed that the capacitor included in the secondcell and the capacitor included in the sixth cell have capacitance2C_(O) and that the capacitor included in the third cell and thecapacitor included in the seventh cell have capacitance 4C_(O). Also, itis assumed that the capacitor included in the fourth cell and thecapacitor included in the eighth cell have capacitance 8C_(O) and thatthe gain of the difference is controlled by the digital gain controlsignal of two bits. In the conventional gain controller, in order tocontrol the gain of the difference to be ‘1’, a digital gain controlsignal ‘00’ is received and the capacitors included in the first andfifth cells are selected. Also, in the conventional gain controller, inorder to control the gain of the difference to be ‘2’, a digital gaincontrol signal ‘01’ is received and the capacitors included in thesecond and sixth cell are selected. In the conventional gain controller,in order to control the gain to be ‘4’, a digital gain control signal‘10’ is received and the capacitors included in the third and seventhcells are selected. Also, in the conventional gain controller, in orderto control the gain to be ‘8’, a digital gain control signal ‘11’ isreceived and the capacitors included in the fourth and eighth cells areselected. In this case, the feedback gain β of the gain controller shownin FIG. 1 becomes 1/16. This is because C_(F) is C_(O) and C_(I) is15C_(O) in Equation 2. Also, in this case, the feedback factor is ‘16’.

FIG. 3 is a bode plot for describing the operation of the gaincontroller shown in FIG. 1. The horizontal axis denotes frequency andthe vertical axis denotes open loop gain represented by dB. Theoperational amplifier 10 shown in FIG. 1 has the frequencycharacteristic 20 as shown in FIG. 3. The operation frequency of thegain controller shown in FIG.1 is f−_(3dB), which is a point where thefeedback factor 1/β (illustrated by reference numeral 22) of the gaincontroller shown in FIG. 1 crosses the frequency characteristic 20 ofthe gain controller. Here, the operation frequency f−_(3dB) increases asthe feedback factor 1/β decreases. Also, as the operation frequencyf−_(3dB) increases, the operating speed of the gain controllerincreases. However, in the conventional gain controller shown in FIG. 1,the feedback factor is ‘16’. Accordingly, the operating speed is not asfast.

In particular, when the gain must be controlled to be precise over awide range, only two corresponding cells must be selected and thecapacitances of the capacitors included in the selected cells must beincreased. Therefore, the entire sampling capacitance, for example,C_(O)+2C_(O)+4C_(O)+8C_(O) increases. Accordingly, the feedback gain isexponentially reduced. As the feedback gain is exponentially reduced,the feedback factor increases. The operating speed is significantlyreduced as the feedback factor increases as shown in FIG. 3.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a gain controllerusing switched capacitors, capable of very quickly controlling the gainof an input signal with low power consumption.

To achieve the above object, there is provided a gain controller whichis operable in a sampling mode or an amplifying mode and which controlsthe gain of an analog input signal. The gain controller of the inventionincludes an operational amplifier, input capacitors, a feedbackcapacitor and a plurality of switches. The operational amplifiercontrols the gain of the analog input signal and generates an outputsignal having the controlled gain. The input capacitors are connected inparallel to the input side of the operational amplifier. The feedbackcapacitor is connected between the input side and the output side of theoperational amplifier. The switches connect at least one of the inputcapacitors to the input signal or a reference voltage, in response tothe kinds of operation modes and an externally applied predetermineddigital gain control signal.

In one embodiment, each of the switches comprises at least onetransistor.

In one embodiment, the gain of the operational amplifier is expressed asa ratio of an input capacitance to the capacitance of the feedbackcapacitor. The input capacitance is the sum of the capacitances of theinput capacitors that are connected to the input signal.

In one embodiment, the digital gain control signal is generated so thatthe desired input capacitance is determined by the combination of thecapacitances of corresponding capacitors among the input capacitors.

The gain controller can be included in a charge-coupled interface systemof an image processing system. The interface system can include acharge-coupled device for generating image data, i.e., photographing animage. A correlated double sampler (CDS) samples and holds thephotographed image and outputs the sampled and held image. The systemalso includes an analog-to-digital converter. The gain controllerdifferentially inputs the output of the CDS and controls the gain of theoutput of the CDS in response to the digital gain control signal. Thedigital gain control signal is a digital signal processed resultcorresponding to the level of a digital signal. The analog-to-digitalconverter converts an analog input signal whose gain is controlled bythe gain controller into the digital signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic block diagram of a conventional gain controller.

FIG. 2 shows waveforms of first and second control clock signals forcontrolling the turning on and off of the MOS transistors shown in FIG.1.

FIG. 3 is a bode plot for describing the operation of the gaincontroller shown in FIG. 1.

FIG. 4 is a schematic circuit diagram of one embodiment of a gaincontroller using switched capacitors according to the present invention.

FIG. 5 is a graph for comparing the frequency characteristic of theconventional gain controller with the frequency characteristic of thegain controller according to the present invention.

FIG. 6 is a graph for comparing the settling time of the conventionalgain controller with the settling time of the gain controller accordingto the present invention.

FIG. 7 is a schematic block diagram of an image processing system usingthe gain controller shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 is a circuit diagram of one embodiment of the gain controllerusing switched capacitors according to the present invention. The gaincontroller includes an operational amplifier 30, first through Nthswitches 40, 42, . . . , 44 and 46, (N+1)th through 2Nth switches 50,52, . . . , 54 and 56, first through Nth capacitors C₁ through C_(N),(N+1)th through 2Nth capacitors C_(N+l) through C_(2N), first and secondfeedback capacitors C_(F3) and C_(F4), and (2N+1)th, (2N+2)th, and(2N+3)th switches 32, 34, and 36.

Referring to FIG. 4, a differential-type or single-type analog signalwhose gain is to be controlled is input through a first input terminalIN1 and a second input terminal IN2. Here, the differential-type meansthat a voltage of +3 volts, for example, is input through the firstinput terminal IN1 and that a voltage of −3 volts is input through thesecond input terminal IN2. Here, the single-type means that the voltageof 3 volts is input through the first input terminal IN1 and a referencevoltage of 0 volts is input through the second input terminal IN2.

The operational amplifier 30 has the frequency characteristic 20 shownin FIG. 3 and includes a first output terminal V_(OUTT) and a secondoutput terminal V_(OUTC) from which the input signal whose gain iscontrolled is output. One side of each of the first through Nthcapacitors C₁ through C_(N) is connected to the negative input terminalof the operational amplifier 30. One side of each of the (N+1)th through2Nth capacitors C_(N+1) through C_(2N) is connected to the positiveinput terminal of the operational amplifier 30. Also, the first throughNth switches 40, 42, . . . , 44, and 46 selectively connect the otherside of each of the first through Nth capacitors C₁ through C_(N),respectively, to the first input terminal IN1 or a ground voltage inresponse to first through Nth selection signals S₁ through S_(N),respectively. Here, each of the switches 40 through 46 can be realizedby at least one transistor. The (N+1)th through 2Nth switches 50, 52, .. . , 54, and 56 selectively connect the other side of each of the(N+1)th through 2Nth capacitors C_(N+1) through C_(2N), respectively, tothe second input terminal IN2 or the ground voltage in response to thefirst through Nth selection signals S₁ through S_(N), respectively.

For example, the first switch 40 connects the other side of the firstcapacitor C1 to the ground voltage or the first input terminal IN1 inresponse to the first selection signal S₁. The (N+1)th switch 50connects the other side of the (N+1)th capacitor C_(N+1) to the groundvoltage or the second input terminal IN2 in response to the firstselection signal S₁.

The first feedback capacitor C_(F3) is connected between the firstoutput terminal V_(OUTT) of the operational amplifier 30 and thenegative input terminal of the operational amplifier 30. The secondfeedback capacitor C_(F4) is connected between the second outputterminal V_(OUTC) of the operational amplifier 30 and the positive inputterminal of the operational amplifier 30. The (2N+1)th and (2N+2)thswitches 32 and 34 connect the negative and positive input terminals,respectively, of the operational amplifier 30 to the reference voltagein a switching mode. Also, the (2N+3)th switch 36 connects the firstoutput terminal V_(OUTT) to the second output terminal V_(OUTC) in theswitching mode.

The gain controller according to the present invention, which is shownin FIG. 4, operates in the sampling mode or in the amplifying mode likethe conventional gain controller shown in FIG. 1. Therefore, the firstthrough Nth selection signals S₁ through S_(N) are determined accordingto the operation mode of the gain controller and the digital gaincontrol signal applied from the outside. Namely, the capacitor isconnected to the first or second input terminal IN1 or IN2 only when thefirst control clock signal Q1 shown in FIG. 4 is at a logic “high” leveland the digital gain control signal is at the logic “high” level, in thesampling mode. In other cases, the first through Nth selection signalsS1 through SN are generated so that the capacitor is connected to theground voltage. Also, in the amplifying mode, the first through Nthselection signals S₁ through S_(N) are generated so that all thecapacitors C₁ through C_(2N) are connected to the ground voltageregardless of the digital gain control signal when the second controlclock signal Q2 (not shown) is at the logic “high” level. Though notshown in detail, the second control clock signal Q2 is considered to bethe same as Q2 shown in FIG. 1, assuming that each of the switches 40through 46 is realized by at least one transistor. At this time, thefirst control clock signal Q1 shown in FIG. 3 is generated so that the(2N+1)th through (2N+3)th switches 32, 34, and 36 are turned on in thesampling mode and are turned off in the amplifying mode.

As assumed in the description of the gain controller shown in FIG. 1, itis assumed that the gain controller according to the present invention,which is shown in FIG. 4, controls the gain of an input voltageV_(INI)−V_(IN2) (V_(IN1) and V_(IN2) denote a voltage input through thefirst input terminal IN1 and a voltage input through the second inputterminal IN2, respectively) to be 1, 2, 4, or, 8. Also, it is assumedthat N is 8 and that the capacitances of all the capacitors C₁ throughC₁₆, C_(F3), and C_(F4) are C_(O). At this time, in the amplifying mode,in the gain controller shown in FIG. 4, only the first and ninthcapacitors C₁ and C₉ are connected to the first and second inputterminals IN1 and IN2 in response to the first through eighth selectionsignals S₁ through S₈ generated corresponding to the logic combinationof a digital gain control signal ‘00’ and the second control clocksignal Q2 (not shown) of the logic “high” level, which is shown in FIG.3, in order to control the gain of the input voltage V_(IN1)−V_(IN2) tobe 1. At this time, the remaining capacitors C₂ through C₈ and C₁₀through C₁₆ are connected to the ground voltage. Therefore, an inputsignal V_(INT−V) _(INC) is sampled only in the first and ninthcapacitors C1 and C9 having the capacitance C_(O). Also, in order tocontrol the gain to be 2, only the first and second capacitors C1 and C2are connected to the first input terminal IN1 in response to the firstthrough eighth selection signals S1 through S8 generated correspondingto the logic combination of a digital gain control signal ‘01’ and thesecond control clock signal Q2 of the logic “high” level. At this time,only the ninth and tenth capacitors C₉ and C₁₀ are connected to thesecond input terminal IN2 and the remaining capacitors C₃ through C₈ andC_(I1) through C₁₆ are connected to the ground voltage. In order tocontrol the gain to be 4, only the first through fourth capacitors C₁through C₄ are connected to the first input terminal IN1 and the ninththrough twelfth capacitors C₉ through C_(I2) are connected to the secondinput terminal IN2 in response to the first through eighth selectionsignals S₁ through S₈ generated corresponding to the logic combinationof a digital gain control signal of ‘10’ and the second control clocksignal Q2 of the logic “high” level. At this time, the remainingcapacitors C₅ through C₈ and C₁₃ through C₁₆ are connected to the groundvoltage. Also, in order to control the gain to be 8, the first througheighth capacitors C₁ through C₈ are connected to the first inputterminal IN1 and the ninth through sixteenth capacitors C₉ through C₁₆are connected to the second input terminal IN2 in response to the firstthrough eighth selection signals S₁ through S₈ generated correspondingto the logic combination of a digital gain control signal ‘11’ and thesecond control clock signal Q2 of the logic “high” level.

In the above-mentioned example, the feedback gain of the gain controlleraccording to the present invention, which is shown in FIG. 4, is 1/9 andthe feedback factor is 9. Therefore, the operation frequency f−_(3d1)shown in FIG. 3 becomes larger than the operation frequency of the gaincontroller shown in FIG. 1. Accordingly, the operating speed of the gaincontroller according to the present invention, which is shown in FIG. 4,is higher than the operating speed of the conventional gain controller.Also, since the sampling capacitance of the gain controller according tothe present invention is 8, which is smaller than the samplingcapacitance of the conventional gain controller invention, that is, 16,it is possible to reduce a chip area when the gain controller isrealized as a chip. Also, the gain of the input signal can be controlledto have more values using limited capacitors since the capacitors areselectively connected to the input voltage by the digital gain controlsignal.

When the digital gain control signal is made from four bits, N is 16,and the capacitances of all the capacitors C₁ through C₃₂, C_(F3), andC_(F4) are C_(O), the gain of the input signal controlled by the gaincontroller shown in FIG. 4 can be generalized as shown in Table 1.

TABLE 1 Gain control signal Capacitor segment combination logic 0000 C₁→a¹C_(O) 0001 C₁ + C₂ →a²C_(O) . . . . . . 1110 C₁ + C₂ + . . . + C₁₅→a¹⁵C_(O) 1111 C₁ + C₂ + . . . + C₁₅ + C₁₆ →a¹⁶C_(O)

When the gain of the input signal is to be controlled as a¹, a², . . . ,a¹⁵, and a¹⁶, the conventional gain controller shown in FIG. 1 providescapacitors having capacitances a¹C_(O), a²C_(O), . . . , a¹⁵C_(O), anda¹⁶C_(O) corresponding to the respective gains and selects one among thecapacitors. However, in the gain controller according to the presentinvention, which is shown in FIG. 4, it is possible to obtain a desiredgain by combining the capacitors C₁ through C_(2N) by the digital gaincontrol signal as shown in Table 1. Therefore, in the present invention,capacitors have capacitances smaller than the capacitances of thecapacitors used in the conventional gain controller.

Namely, it is possible to obtain a large gain using a capacitor having alarge capacitance in the conventional gain controller. However, it ispossible to obtain a large gain by combining capacitors having smallcapacitances, in the present invention.

FIG. 5 is a graph comparing the frequency characteristic of theconventional gain controller with the frequency characteristic of thegain controller according to the present invention. The vertical axisdenotes gain in units of dB and the horizontal axis denotes frequency.

Referring to FIG. 5, while the operating frequency f−_(3dB) A of theconventional gain controller is 10.7 MHZ, the operating frequencyf−_(3dB) B of the gain controller according to the present invention is77.7 MHZ, which is about 7 times greater than the conventional operatingfrequency.

FIG. 6 is a graph comparing the settling time of the conventional gaincontroller with the settling time of the gain controller according tothe present invention. The vertical axis denotes the gain in units of dBand the horizontal axis denotes the time.

Referring to FIG. 6, a process A of settling the gain of the inputsignal to be a desired value ‘1’ very slowly proceeds in theconventional gain controller shown in FIG. 1. While the settling time isvery long in the conventional gain controller, a process B of settlingthe gain of the input signal to be the desired value ‘1’ very rapidlyproceeds in the gain controller according to the present invention ofFIG. 4. It is noted that the settling time is very short in the gaincontroller according to the present invention. Therefore, since thesettling time is short, the power consumption of the gain controlleraccording to the present invention is less than the power consumption ofthe conventional gain controller.

The gain controller according to the present invention, which is shownin FIG. 4, can be applied to image processing systems in which a smallarea and small power consumption are required, such as scanners,portable camcorders, and digital still cameras. An example will now bedescribed with reference to the attached drawings.

FIG. 7 is a schematic block diagram of an image processing system usingthe gain controller shown in FIG. 4. Referring to FIG. 7, the imageprocessing system includes a charge coupled interface system 80 adigital signal processor 100, a loop gain unit 102, and an integrator104.

The charge coupled interface system 80 shown in FIG. 7 includes acorrelated double sampler (CDS) 82 made of two sample and hold units 90and 92, a gain controller 84 corresponding to the gain controller shownin FIG. 4, and an analog-to-digital converter 86.

Referring to FIG. 7, a charge coupled device (CCD) (not shown)photographs an image. A video signal generated by the CCD (not shown),that is, a CCD video signal including a reference level REF and a datalevel DATA, is output to the CDS 82. The CDS 82 samples and holds theCCD video signal from the CCD (not shown) on the basis of a common levelCOMMON, thus removing noise. The result of sampling and holding isoutput to the gain controller 84. The analog-to-digital converter 86receives a result where the gain is controlled from the CDS 82, convertsthe received result into a digital signal, and outputs the converteddigital signal.

When the level of a signal input to the CCD (not shown) suddenlyincreases or decreases due to variations in brightness of an image inputfrom the outside, the signal deviates from the input range of theanalog-to-digital converter 86 or concentrates on very low codes. Whenthe converted digital code is reproduced to an image in such a state,the image is too bright or too dark. Therefore, it is difficult torecognize the image.

Therefore, the digital signal processor 100 analyzes and processes thelevel of the digital signal output from the analog-to-digital converter86. The result analyzed and processed by the digital signal processor100 is the digital gain control signal, which is output to the gaincontroller 84 and the loop gain unit 102. The gain controller 84, whichcan be realized by the circuit shown in FIG. 4, controls the gain of theresult of sampling and holding, which is differentially received fromthe CDS 82 in response to the digital gain control signal. Therefore,the magnitude of the signal output from the analog-to-digital converter86 can be increased or reduced to an appropriate level. Here, theplurality of gain controllers 84 can be serially connected between theCDS 82 and the analog-to-digital converter 86 unlike in FIG. 7.Accordingly, it is possible to obtain a desired gain.

The image processing system shown in FIG. 7 can operate in two modes. Afirst mode is a black-level correction period coinciding with the levelof the CCD video signal generated from real black light with the lowestinput level of the analog-to-digital converter 86. In the case of atypical digital camcorder, the black-level correction period is repeatedevery 2 μs during a period which repeats every 64 μs. During thisperiod, the CCD outputs the level generated by the black light, which isa signal from a pixel covered with a metal, to the CDS 82. At this time,a negative feedback signal which passes through the loop gain unit 102and the integrator 104 begins to be processed. In order to make theblack-level coincide with the lowest reference voltage Vref−Vref⁺ of theanalog-to-digital converter 86, the loop gain unit 102 compares theoutput of the gain controller 84 with the lowest reference voltage usedby the analog-to-digital converter 86. Therefore, the loop gain unit 102attenuates or amplifies the difference generated by the comparisonresult and outputs the attenuated or amplified difference to theintegrator 104. The integrator 104 continuously accumulates the signalreceived from the loop gain unit 102 and outputs the accumulated outputto the CDS 82. The CDS 82 subtracts the output of the integrator 104from the input signals REF and DATA and transmits the subtraction resultto the gain controller 84. The output of the gain controller 84generated by the input of the black-level signal is converged to andfixed to the lowest reference voltage of the analog-to-digital converter86, due to the negative feedback operation.

In a second mode, the loop gain unit 102 on a negative feedback signalpath during a period where the video signal is processed does notoperate and the integrator 104 continuously supplies the outputdetermined during the black-level correction period to the CDS 82. Afterthe black-level is fixed, when a general video signal is processed,picture information which passed only through the CDS 82 and the gaincontroller 84 is output to the analog-to-digital converter 86.

As mentioned above, it is possible to automatically control the gain ofthe input signal at high operating speed and to reduce power consumptionsince the time required for settling the gain of the input signal to adesired value by the gain controller using the switched capacitoraccording to the present invention is short. Also, according to the gaincontroller of the present invention, it is possible to control the gainof the input signal to have more values than in the conventional gaincontroller, using a limited number of capacitors. The gain controlleraccording to the present invention can be realized by a chip having asmaller size since less sampling capacitance is required for obtainingthe desired gain than in the conventional gain controller.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the following claims.

What is claimed is:
 1. A gain controller for controlling gain of ananalog input signal, said gain controller being operable in a samplingoperational mode and an amplifying operational mode, comprising: anoperational amplifier for controlling the gain of the analog inputsignal and generating an output signal having the controlled gain, theoperational amplifier being operable in a switching mode and having anoutput which includes first and second output terminals, an outputterminal switch being operable to connect the first and second outputterminals together when the operational amplifier is in the switchingmode; input capacitors connected in parallel to the input side of theoperational amplifier; a feedback capacitor connected between the inputside and the output side of the operational amplifier; a plurality ofswitches for selectively connecting at least one of the input capacitorsto at least one of the input signal and a reference voltage, in responseto the operational mode and an externally applied predetermined digitalgain control signal; an input which differentially inputs an output of acorrelated double sampler (CDS), the CDS sampling and holding aphotographed image and providing the sampled and held image as theoutput of the CDS; gain controlling circuitry which controls the gain ofthe output of the CDS in response to a digital gain control signal, thedigital gain control signal being provided by a digital signal processorin response to a digital signal; and an output which provides again-controlled output signal to an analog-to-digital converter whichconverts the gain-controlled/output signal into the digital signal. 2.The gain controller of claim 1, wherein each of the switches comprisesat least one transistor.
 3. The gain controller of claim 1, wherein thegain of the operational amplifier is expressed as a ratio of an inputcapacitance to the capacitance of the feedback capacitor, the inputcapacitance being the sum of the capacitances of the input capacitorsthat are connected to the input signal.
 4. The gain controller of claim1, wherein the digital gain control signal is generated so that thedesired input capacitance is determined by the combination of thecapacitances of corresponding capacitors among the input capacitors.